When plotters dream of a caretaker PM, they tend to dream of Johnson. The home secretary is popular, admired by the opposition, has a gritty backstory and is at least interested in being prime minister. But he has doubted himself and others wonder whether he can move votes during an election campaign. Has not been an unqualified success as home secretary (who is?) Shallowness of support within Labour shown when he lost the deputy leadership contest.

Odds: 9-4 favourite

Jon Cruddas, backbencher

Cruddas's real views on the leadership are shrouded in secrecy. He is possibly the favourite to be deputy leader, in a run of events that would see Brown stand down after a general election defeat. Would be pressed to stand for the leadership too as a clean-slate, fresh-start candidate. Has a broad constituency embracing a lot (but by no means all) of the left, ex-Blairites such as James Purnell and thinkers across and beyond the party. Has never been a minister.

Odds: 7-2

David Miliband, foreign secretary

Virtually a certain candidate in any contest. Conspicuous by his absence from the airwaves as the plot unfolded today. Torn about whether to stay or go when his friend James Purnell quit last year. Intellectually he is ahead of his rivals, but characterised as a bottler – which is unfair, say his friends – and has less support in the parliamentary Labour party than he should have. That may change with a new intake.

Odds: 4-1

Ed Miliband, climate change secretary

The man who some think makes his elder brother not even the best candidate in his own family, never mind the Labour party. Behind-the-scenes the youngish (at 40) former Brown aide has cultivated union leaders including Derek Simpson of Unite for years. His environmental credentials make him popular with the rank and file. But very reluctant to move against Brown before an election; won't want a punch-up with his brother either.

Odds: 6-1

Harriet Harman, deputy leader

A decent outside bet to replace Gordon Brown, whenever that happens: Labour's biggest name other than the prime minister, with a legislative and campaigning record that puts most of her colleagues in the shade. But most of the media dislike her. Unlikely to move against Brown – but if she did she could help topple him.

Odds: 15-2

Ed Balls, schools secretary

The prime minister's closest political ally was the first cabinet minister to be seen at No 10 after the plot surfaced – although he was apparently on his way to meet the Fonz, Henry Winkler, at an event hosted by Brown. Balls would definitely stand in a leadership contest if Brown resigned. Loyalty and ambition are in conflict: he would have a better chance of becoming leader before the election than after, on a coalition of old left and Brown loyalists. If he told Brown he had to go, he would have to go.

Odds: 25-1

Tipping points

The events that could put Gordon Brown in trouble again:

• Another backbench insurrection: a new letter, a new petition, or a stormy ride for the prime minister at next Monday's parliamentary party meeting

• The left turns. So far those MPs in and around the Compass pressure group have stayed loyal. If that changed it would help dilute the perception of a Blairite plot

• Ministers resign: Brown suffered from the drip-drip of ministerial resignations in and around last year's local elections. He could survive others – probably – if they were outside the cabinet

• More bad news. Penguin Books promises "astonishing revelations" in a new book by Andrew Rawnsley, below, on the Blair-Brown years: enough to unseat the prime minister?

• Cabinet heavyweights do a "Geoffrey Howe": if either David Miliband, Alistair Darling, Lord Mandelson or Jack Straw tell him he has to go, Brown would almost certainly be fatally damaged

• Friends turn against him. Were Ed Balls, Yvette Cooper and Ed Miliband to tell the prime minister his time his up, he would be forced to resign immediately